1. Field of the Invention
This invention relates to a method and apparatus for making continuous grids for lead acid batteries.
2. Description of the Prior Art
It is generally known that grids for lead acid batteries are made by casting lead or a lead alloy. It has also been recently considered to make grids by punching or expanding a narrow strip of lead or a lead alloy obtained by continuous casting or rolling in advance.
In the method by casting, skilled work is required in a hot environment, the productivity is very low and it is very difficult to make thin grids required for high performance batteries. In the method by punching, there are defects in that a large amount of punching scraps will be produced, a large labor and cost will be required to recover and refine them and material will be wasted. In this respect, in making grids by expanding, substantially no scrap will be produced, the yield rate of the material is high and there is a great advantage that grids can be made thin and light.
However, in the conventional method wherein zigzag slits are formed in a narrow strip in the direction at right angles with the advancing direction of the strip and the strip is expanded in the advancing direction, there have been defects in that it is difficult to form such unslitted portions as the current collecting header, bottom border and lug required for grids of lead acid batteries. Also if such unslitted portions are to be obtained, such that the strip must be worked so as to form no zigzag slit at fixed intervals, the work and apparatus will be complicated and the productivity will be very low.
In order to cope with the above mentioned defects, there is recently considered a producing method wherein zigzag slits parallel with the advancing direction of a narrow strip are formed in the strip and the strip is extended in the direction at right angles with the advancing direction or in the direction vertical to the surface of the strip. In this method, there is a step wherein zigzag slits parallel to the advancing direction of a strip are formed in the strip and then the strip is pulled and extended in the direction at right angles with the advancing direction while being held at the edge portions thereof. However, in this method, there are defects in that errors are likely to be produced in the extended dimension and the respective grid dimensions of the obtained product are hard to be made uniform. On the other hand, there is also considered a method wherein a different die and cutter are provided for each row of slits, a narrow strip is intermittently fed in turn and the die is intermittently driven in the vertical direction to form slits in turn from the edge portions of the strip and, at the same time, the strip is extended along the shape of the die in the vertical direction. However, in this method, there are defects in that the apparatus is complicated and large and that, if the precision of the intermittent feed of the strip is low even slightly, the rows of slits formed with the respective dies and cutters will be likely to become loose and it will be difficult to obtain precision grid cells.
Further, on the other hand, there are defects that the grids made by expanding will be likely to be disconnected by the corrosion and growth during battery use and will be short lived. Particularly, a positive grid made of a lead-calcium alloy will have remarkably high growth during the battery charge-discharge but, as it is fixed in the container of the battery, it will be disconnected early at the connecting portion between the wire-like segments and will have reduced current collecting capacity and will be short lived in most cases.
Further, a grid made by expanding has defects that, as its surface or slitted surface is smoother than the cast surface of a grid made by casting, its area of contact with the pasted active material will be smaller than that of the grid made by casting, therefore the retaining force will be weak, as a result, the current collecting capacity will be low, the active material will shed and the life of the battery will be short.